This application claims the priority benefit of Taiwan application serial no. 90118009, filed Jul. 24, 2001.
1. Field of the Invention
The invention relates to a polishing machine. More particularly, the present invention relates to a pressure regulation system used in a polishing machine.
2. Description of the Related Art
In semiconductor manufactures, integrated circuits are conventionally formed on substrates, particularly silicon wafers, by the successive depositions of conductive, insulative, or semiconductive layers. After a layer is deposited, the layer generally is etched to remove material from selected regions to create the desired circuitry features. As the number of deposited and etched layers increases, the topmost surface of the substrate successively becomes less planar because the distance between the topmost surface and the underlying substrate is the greatest at the least etched regions while it is the least at the greatest etched regions.
A non-planar upper surface is problematic when a photolithography is to be performed to pattern a layer deposited over the substrate. For example, the accuracy of the pattern transfer onto the layer critically depends on the planarity of the upper surface of the layer and is ensured only if the layer surface is not irregular, which otherwise may scatter the light during exposure. Therefore, the surface of the substrate needs to be periodically planarized to provide a relatively flat and smooth layer surface. Polishing methods such as chemical mechanical polishing method are methods known in the art.
Referring to FIG. 1, a simplified diagram schematically shows a conventional polishing machine. The structure of a conventional polishing machine comprises a wafer carrier head 10 rotary and slidably mounted, a platen 18 rotary mounted, and a polishing pad 22 fixedly arranged on the platen 18. To perform a polishing, a substrate, for example a wafer, is fixedly mounted on the wafer carrier head 10 by means of for example an adhesive layer 20. By means of a pressure P1 and a pressure P2, the platen 18 and the wafer carrier head 10 respectively press the wafer against each other. While the to be-planarized surface of the wafer is thus pressed against the polishing pad 22, the wafer carrier head 10 and/or the platen 18 move relative to each other to generate a relative motion between the wafer and the polishing pad 22. During polishing, a polishing slurry including an abrasive suspended in a liquid and at least one chemically-reactive agent for chemical mechanical polishing is regularly applied onto the polishing pad 22 to provide an abrasive and chemically reactive mixture at the wafer-polishing pad interface.
To obtain an adequate polishing of the wafer, many factors such as the relative speed between the polishing pad and the wafer, the total polishing time, and the pressure applied during polishing must be considered. With respect to the control of the pressure applied during polishing, various specific structures of the wafer carrier head are known in the art.
U.S. Pat. No. 5,584,751 issued to Kobayashi et al. discloses a wafer carrier head that improves the polishing uniformity by applying various pressures to a wafer carrier head. In U.S. Pat. No. 5,584,751, a first pressure applied to a diaphragm presses a wafer carrier holding a wafer against a polishing pad while a second pressure is applied to a retainer ring that presses against the polishing pad at an outer periphery of the wafer.
U.S. Pat. No. 6,143,123 issued to Robinson et al. discloses a polishing machine that includes a pressure sensor embedded in the polishing pad to measure the pressure at various areas of the surface of the wafer being polished. Via the sensing of the pressure, a plurality of actuators adjust an adequate pressure during the polishing.
By means of various technical arrangements, these patents provide improvements of the polishing by emphasizing one aspect: the pressure applied during polishing. However, the prior art references neither disclose nor solve an overshoot problem that occurs when the wafer is pressed between the polishing pad and the wafer carrier head, as described hereafter. Still with reference to FIG. 1 and as described above, to perform a planarization, the wafer is pressed between the wafer carrier head 10 and the platen 18 by means of first and second pressures P1 and P2 respectively applied on the platen 18 and the wafer carrier head 10. Practically, a tight maintain of the wafer is ensured only at the condition that the first pressure P1 is greater than the second pressure P2 within an adequate range, in other words the difference of pressure xcex94P=P2xe2x88x92P1 less than 0. During a planarization operation, the operator thus sets the first and second pressures P1 and P2 such that the difference of pressure xcex94P is constantly equal to a predetermined negative value. However, before attaining a steady state where xcex94P is constant, a relatively high peak overshoot usually occurs during a transient response of xcex94P. This overshoot means an excessive difference of pressure xcex94P that may damage the wafer and cause instability of the pressure regulation system.
A major aspect of the present invention is to provide a controlled pressure regulation system for polishing machine and a method for regulating the wafer pressing pressures in a polishing machine that prevents damages of the wafer to be polished.
To accomplish at least the above objectives, the present invention provides a controlled pressure regulation system that comprises the following elements. A wafer carrier head holds a wafer to be polished against a platen. A first pressure regulator generates a first pressure onto the platen and a second pressure regulator generates a second pressure onto the wafer carrier head to press the wafer to be polished between the platen and the wafer carrier head. A first controller is connected to the first pressure regulator in a first feedback loop to control the generation of the first pressure onto the platen. A second controller is connected to the second pressure regulator in a second feedback loop to control the generation of the second pressure onto the wafer carrier head according to the difference between the difference between the first pressure and the second pressure. The control of the generation of the first and second pressures, preferably performed by proportional integral controllers, prevents peak overshoot of the difference of pressure between the first pressure and second pressure, which consequently prevents damages of the wafer to be polished.
The present invention further provides a method of pressure regulation applied during a polishing to press a wafer to be polished between a wafer carrier head and a platen. The method comprises the following steps. A first pressure is generated onto the platen. The generation of the first pressure onto the platen is controlled by a first control feedback loop. A second pressure is generated onto the wafer carrier head. The generation of the second pressure is controlled according to a difference of pressure between the first and second pressure via a second control feedback loop.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.